Vehicle motion control system

ABSTRACT

In a vehicle motion control system based on a target yaw rate scheme, a brake force computing unit brakes an outer front wheel in a controlled manner when a sign of the yaw rate has changed and the yaw rate increment has become equal to or greater than a threshold value after a counter steer action has been detected and the vehicle body slip angle has become equal to or greater than a threshold value. Thus, suppose that a vehicle travels a winding road and a counter steer action is taken. If the vehicle body slip angle and actual yaw rate increment exceed threshold values, the outer front wheel is braked. Therefore, even when the vehicle body slip angle reaches a maximum value and an attempt is made to reduce it again by a counter steer action in a similar manner as a swinging pendulum, because the increases in the yaw rate and actual yaw rate increment are predicted and monitored before the vehicle body slip angle reaches its maximum value, the vehicle is enabled to travel in a stable fashion.

TECHNICAL FIELD

[0001] The present invention relates to a vehicle motion control systemfor controlling the motion of a vehicle in a stable manner even when itis steered successively in opposite directions.

BACKGROUND OF THE INVENTION

[0002] Conventionally known is a vehicle motion control system whichdetermines a dynamic variable of the vehicle as the vehicle turns acorner and controls the motion of the vehicle according to the deviationof the dynamic variable from a target value of the dynamic variable. Forinstance, according to a known yaw rate control system, a deviation of ayaw rate actually measured by using a yaw rate sensor from a target yawrate computed from the steering angle is converted into a yaw momentthat would eliminate the deviation, and the right and left wheels arebraked individually and differently so as to produce a corresponding yawmoment. For instance, when the vehicle has an oversteer tendency, theouter wheels are braked more than the inner wheels. Conversely, when thevehicle has an understeer tendency, the inner wheels are braked morethan the outer wheels.

[0003] However, according to such a conventional vehicle motion controlsystem, when the vehicle is brought back from a state involving a largeslip angle to a stable state either by the intervention of the vehicleoperator or automatically by the system, an overshoot in the yawmovement of the vehicle tends to be produced due to the inertia of thevehicle, and even an oscillatory yaw movement may result.

BRIEF SUMMARY OF THE INVENTION

[0004] In view of such problems of the prior art, a primary object ofthe present invention is to provide a vehicle motion control systembased on a target yaw rate scheme which ensures a stable motion to thevehicle under all conditions.

[0005] A second object of the present invention is to provide a vehiclemotion control system based on a target yaw rate scheme which ensures astable motion to the vehicle without requiring any significant change tothe existing system.

[0006] According to the present invention, these and other objects ofthe present invention can be accomplished by providing a vehicle motioncontrol system for controlling a motion of a vehicle during cornering,comprising: a steering angle sensor for detecting a steering angle; ayaw rate sensor for detecting an actual vehicle yaw rate; an actual yawrate increment computing unit for computing an increment of the actualyaw rate; a vehicle body slip angle computing unit for estimating avehicle body slip angle; a brake force computing unit for controllingbrake forces of right and left front wheels of the vehicle; and acounter steer detecting unit for detecting a counter steer actionaccording to the steering angle and yaw rate; the brake force computingunit being adapted to brake an outer front wheel in a controlled mannerwhen a sign of the yaw rate has changed and the yaw rate increment hasbecome equal to or greater than a threshold value after a counter steeraction has been detected and the vehicle body slip angle has becomeequal to or greater than a threshold value.

[0007] Thus, suppose that a vehicle travels a winding road and a countersteer action is taken. If the vehicle body slip angle and actual yawrate increment exceed threshold values, the outer front wheel is braked.Therefore, even when the vehicle body slip angle reaches a maximum valueand an attempt is made to reduce it again by a counter steer action (ina similar manner as a swinging pendulum), because the increases in theyaw rate and actual yaw rate increment are predicted and monitoredbefore the vehicle body slip angle reaches its maximum value, thevehicle is enabled to travel in a stable fashion.

[0008] Preferably, the counter steer detecting unit is adapted to detecta counter steer action when signs of the steering angle and yaw ratediffer from each other.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Now the present invention is described in the following withreference to the appended drawings, in which:

[0010]FIG. 1 is a diagram showing the overall structure of the vehiclemotion control system embodying the present invention;

[0011]FIG. 2 is a block diagram showing the estimation logic for variouscontrol values;

[0012]FIG. 3 is a diagram showing the definition of tire slip angles,tire lateral forces, vehicle body lateral acceleration, tirefore-and-aft forces and yawing motion of a vehicle turning a corner;

[0013]FIG. 4 is a first part of a general flow chart of the controlprocess in the vehicle motion control system;

[0014]FIG. 5 is a subroutine flow chart for computing the estimatedvehicle body speed;

[0015]FIG. 6 is a second part of the general flow chart;

[0016]FIG. 7 is a graph representing the map for obtaining the tirefore-and-aft force coefficient;

[0017]FIG. 8 is a graph representing the map for obtaining the tirelateral force coefficient;

[0018]FIG. 9 is a graph representing the map for obtaining the tirelateral force reduction coefficient;

[0019]FIG. 10 is a subroutine flow chart for computing the estimated yawrate;

[0020]FIG. 11 is a first part of a subroutine flow chart for computingthe road surface frictional coefficient;

[0021]FIG. 12 is a second part of the subroutine flow chart forcomputing the road surface frictional coefficient;

[0022]FIG. 13 is a third part of the subroutine flow chart for computingthe road surface frictional coefficient;

[0023]FIG. 14 is a first part of a subroutine flow chart for computingthe degree of the vehicle momentum control;

[0024]FIG. 15 is a second part of the subroutine flow chart forcomputing the degree of the vehicle momentum control;

[0025]FIG. 16 is a third part of the subroutine flow chart for computingthe degree of the vehicle momentum control;

[0026]FIG. 17 is a block diagram showing the estimation logic forvarious control values in the vehicle motion control;

[0027]FIG. 18 is a diagram showing the control mode at the time ofundersteer/oversteer;

[0028]FIG. 19 is a block diagram showing the vehicle body slip anglecontrol logic;

[0029]FIG. 20 is a block diagram showing the vehicle moment controllogic;

[0030]FIG. 21 is a flow chart of a vehicle motion control according tothe present invention;

[0031]FIG. 22 is a diagram showing the various states of a travelingvehicle; and

[0032]FIG. 23 is a diagram showing the principle of the pendulum controlin connection with FIG. 22.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033]FIG. 1 is an overall system diagram of an automobile to which thepresent invention is applied. This vehicle is fitted with a steeringdevice for steering the front wheels FR and FL which is provided with asteering sensor 1. A yaw rate sensor 2, lateral acceleration sensor 3and fore-and-aft acceleration sensor 4 are placed in appropriatelocations of the vehicle body. Each of the wheels FR, FL, RR and RL isprovided with a wheel speed sensor 5 a, 5 b, 5 c and 5 d for measuringthe rotational speed of the corresponding wheel. These sensors areconnected to a central control unit 6. The central control unit 6 isincorporated with a brake hydraulic pressure control actuator HU forcontrolling the braking force of each individual wheel, and is connectedto an electronic throttle controller DBW for controlling the openingdegree of the engine throttle valve and a PGM-FI controller forcontrolling the fuel injection and ignition timing of the engine. Thecentral control unit 6 is connected to a monitor 7 to allow the statusof the central control unit 6 to be visually displayed.

[0034]FIG. 2 is a block diagram showing the estimation logic for variouscontrol variables used in the central control unit 6 embodying thepresent invention. In the illustrated embodiment, the values detected bythe various sensors 1-4 and 5 a-5 d are used for determining the controlvariables that are needed to execute a vehicle motion control accordingto the present invention. This vehicle motion control is of such a kindthat can achieve both a high stability and a favorable steerability, andcan be extended for the use of rear wheel drive vehicles and four wheeldrive vehicles as well as front wheel drive vehicles. It is essentialfor this vehicle motion control that the slippage of the four wheels areoptimally controlled, and it is accomplished by using a four wheel brakeactuator which can control the braking force of each individual wheel.

[0035] As shown in FIG. 3, the variables that are needed for thiscontrol during cornering include tire slip angles α_(FR), α_(FL), α_(RR)and α_(RL), tire lateral forces CF_(FR), CF_(FL), CF_(RR) and CF_(RL), avehicle body lateral acceleration LGE, and tire fore-and-aft forces orbrake/traction forces FX_(FR), FX_(FL), FX_(RR) and FX_(RL). A roadfrictional coefficient between the tire and road surface is necessaryfor determining a fore-and-aft acceleration FGE as well as thebrake/traction forces and tire lateral forces.

[0036] The steering angle sensor 1 determines a steering angle STC, thewheel speed sensors 5 a-5 d determine the wheel speeds VW_(FR), VW_(FL),VW_(RR) and VW_(RL), the yaw rate sensor 2 determines the yaw rate YAWR,the lateral acceleration sensor 3 determines the vehicle body lateralacceleration LG, and the fore-and-aft acceleration sensor 4 determinesthe vehicle body fore-and-aft acceleration FG.

[0037] The steering angle STC is forwarded to an actual tire steeringangle computing unit STA_(n), and the actual tire steering anglesSTA_(R) and STA_(L) of the right and left front wheels are forwarded toa tire slip angle computing unit α_(mn). The wheel speeds VW_(FR),VW_(FL), VW_(RR) and VW_(RL) are forwarded to a tire slip ratiocomputing unit SLP_(mn), the yaw rate YAWR is forwarded to an actual yawrate increment computing unit ΔYRR, and the vehicle body accelerationsLG and FR are forwarded to a wheel load computing unit FZ_(mn).

[0038] In the illustrated embodiment, the wheel load computing unitFZ_(mn) computes wheel loads FZ_(FR), FZ_(FL), FZ_(RR) and FZ_(RL)according to a dynamic tire model TM and the vehicle body accelerationsLG and FR. The values produced from the wheel load computing unitFZ_(mn) are forwarded to a tire lateral force (cornering force)computing unit CF_(mn) and a tire fore-and-aft force computing unitFX_(mn). Other methods may also be used for computing the wheel loads,for instance by actually measuring the vertical loads of the individualwheels.

[0039] The tire fore-and-aft force computing unit FX_(mn) computes thebrake/traction forces FX_(FR), FX_(FL), FX_(RR) and FX_(RL) of theindividual wheels according to the wheel loads obtained from the wheelload computing unit FZ_(mn), the tire slip ratios SLP_(FR), SLP_(FL),SLP_(RR) and SLP_(RL) of the individual wheels obtained from the tireslip ratio computing unit SLP_(mn), and the road frictional coefficientμ which is computed by a road surface frictional coefficient computingunit μ. The various values produced from the tire fore-and-aft forcecomputing unit FX_(mn) are forwarded to an estimated fore-and-aftacceleration computing unit FGE which then produces an estimatedfore-and-aft acceleration FGE according to the brake/traction forcesFX_(FR), FX_(FL), FX_(RR) and FX_(RL). The estimated fore-and-aftacceleration FGE is filtered by an estimated fore-and-aft accelerationfilter FGEF. The filtered estimated fore-and-aft acceleration FGEF isthen forwarded to the road surface frictional coefficient computing unitμ.

[0040] The filtered estimated fore-and-aft acceleration FGEF is alsoforwarded to an estimated vehicle body speed (X-component) computingunit VVXβ which then computes an estimated vehicle body speed(X-component) VVXβ or a fore-and-aft speed of the vehicle body. Theestimated vehicle body speed (X-component) VVXβ is then forwarded to acontact point speed (X-component) computing unit VCX_(mn).

[0041] The contact point speed (X-component) computing unit VCX_(mn)receives an estimated yaw rate CFYAWR from an estimated yaw ratecomputing unit CFYAWR which is described hereinafter in addition to theestimated vehicle body speed (X-component) VVXβ. The contact point speed(X-component) computing unit VCX_(mn) produces contact point speeds(X-component) VCX_(FR), VCX_(FL), VCX_(RR) and VCX_(RL) as estimatedfore-and-aft wheel speeds of the individual wheels. The contact pointspeeds (X-component) may correspond to the fore-and-aft vehicle bodyspeed at the contact points of the individual wheels.

[0042] The estimated vehicle body speed (X-component) VVXβ and estimatedyaw rate CFYAWR are also forwarded to a contact point speed(Y-component) computing unit VCY_(mn) which additionally receives avehicle body slip angle β from a vehicle body slip angle computing unitβ, in addition to the estimated vehicle body speed (X-component) VVXβand estimated yaw rate CFYAWR, to compute contact point speeds(Y-component) VCY_(FR), VCY_(FL), VCY_(RR) and VCY_(RL) as estimatedlateral wheel speeds of the individual wheels. The contact point speeds(Y-component) may correspond to the lateral body speed at the contactpoints of the individual wheels.

[0043] The various values produced from the contact point speed(X-component) computing unit VCX_(mn) are forwarded to the tire slipangle computing unit α_(mn) and a rolling direction speed computing unitVC_(mn). The various values produced from the contact point speed(Y-component) computing unit VCY_(mn) are also forwarded to the tireslip angle computing unit α_(mn) and rolling direction speed computingunit VC_(mn). In the illustrated embodiment, the tire slip anglesα_(FR), α_(FL), α_(RR) and α_(RL) of the individual wheels are computedaccording to the actual tire steering angle STA_(n), contact pointspeeds (X-component) VCX_(FR), VCX_(FL), VCX_(RR) and VCX_(RL), andcontact point speeds (Y-component) VCY_(FR), VCY_(FL), VCY_(RR) andVCY_(RL) of the individual wheels, and are forwarded to the tirecornering force computing unit FY_(mn) and rolling direction speedcomputing unit VC_(mn). The tire slip angles may also be determined byusing other methods.

[0044] The rolling direction speed computing unit VC_(mn) computesrolling direction speeds VC_(FR), VC_(FL), VC_(RR) and VC_(RL) of theindividual wheels according to the tire slip angles obtained from thetire slip angle computing unit α_(mn) and the values obtained from thecontact point speed (X-component) computing unit VCX_(mn) and contactpoint speed (Y-component) computing unit VCY_(mn). In the illustratedembodiment, the various values produced from the rolling direction speedcomputing unit VC_(mn) are forwarded to the slip ratio computing unitSLP_(mn) which then computes the slip ratios SLP_(FR), SLP_(FL),SLP_(RR) and SLP_(RL) of the individual wheels according to the rollingdirection speeds VC_(FR), VC_(FL), VC_(RR) and VC_(RL) and wheel speedsVW_(FR), VW_(FL), VW_(RR) and VW_(RL). The slip ratios may also bedetermined by using other methods.

[0045] The tire slip angles obtained from the tire slip angle computingunit α_(mn) are also supplied to the cornering force computing unitFY_(mn) which computes the cornering forces FY_(FR), FY_(FL), FY_(RR)and FY_(RL) of the individual wheels according to the tire slip angles.The cornering forces that are produced from the cornering forcecomputing unit FY_(mn) are forwarded to the tire lateral force(cornering force) computing unit CF_(mn).

[0046] The tire lateral force (cornering force) computing unit CF_(mn)receives the slip ratios from the slip ratio computing unit SLP_(mn),the wheel loads from the wheel load computing unit FZ_(mn), and the roadfrictional coefficient μ from the road surface frictional coefficientcomputing unit μ, in addition to the cornering forces, and computes thetire lateral forces CF_(FR), CF_(FL), CF_(RR) and CF_(RL) of theindividual wheels according to these variables. The output from the tirelateral force (cornering force) computing unit CF_(mn) is forwarded toan estimated lateral acceleration computing unit LGE. The estimatedlateral acceleration computing unit LGE computes an estimated lateralacceleration LGE according to the tire lateral forces obtained from thetire lateral force (cornering force) computing unit CF_(mn). Theestimated lateral acceleration LGE is filtered by an estimatedacceleration filter LGEF, and is then forwarded to the aforementionedroad surface frictional coefficient computing unit μ.

[0047] A tire grip force computing unit TGM computes a total grip forceRTGM according to the vehicle body lateral acceleration LG and vehiclebody fore-and-aft acceleration FG. The obtained total grip force RTGM,along with the outputs of the acceleration sensors LG and FG, isforwarded to the road surface frictional coefficient computing unit μ.The total grip force RTGM is computed as a square root of the sum of thesquare of the vehicle body lateral acceleration LG and the square of thevehicle body fore-and-aft acceleration FG, or {(FG²+LG²)^(½)}.

[0048] The actual yaw rate increment computing unit ΔYRR computes anactual yaw rate increment ΔYRR according to the yaw rate sensor valueYAWR, and the computed actual yaw rate increment ΔYRR is then forwardedto a front and rear wheel moment correction coefficient computing unitCFK_(X). The front and rear wheel moment correction coefficientcomputing unit CFK_(X) also receives the tire lateral forces from thetire lateral force computing unit CF_(mn), in addition to the computedactual yaw rate increment ΔYRR, and computes a front and rear wheelmoment correction coefficient CFK1 and CFK2 according to thesevariables. The front and rear wheel moment correction coefficients CFK1and CFK2 are forwarded to an estimated yaw rate increment computing unitΔYRE.

[0049] The estimated yaw rate increment computing unit ΔYRE receives thetire lateral forces from the tire lateral force computing unit CF_(mn),in addition to the front and rear wheel moment correction coefficientsCFK1 and CFK2, and computes an estimated yaw rate increment ΔYREaccording to these variables. The estimated yaw rate increment ΔYRE isforwarded to an estimated yaw rate computing unit CFYAWR.

[0050] The estimated yaw rate computing unit CFYAWR computes anestimated yaw rate CFYAWR according to the estimated yaw rate incrementΔYRE. The estimated yaw rate CFYAWR is forwarded to the contact pointspeed (X-component) computing unit VCX_(mn) and contact point speed(Y-component) computing unit VCY_(mn) as well as to a vehicle body slipangle increment computing unit Δβ.

[0051] An estimated vehicle body speed computing unit VVβ computes anestimated vehicle body speed VVβ according to the estimated vehicle bodyspeed (X-component) VVXβ and vehicle body slip angle β. The estimatedvehicle body speed VVβ, estimated yaw rate CFYAWR and filtered estimatedacceleration LGEF are forwarded to the vehicle body slip angle incrementcomputing unit Δβ. According to these values, the vehicle body slipangle increment computing unit Δβ computes a vehicle body slip angleincrement Δβ. The vehicle body slip angle computing unit β then computesa vehicle body slip angle β according to the vehicle body slip angleincrement Δβ. The vehicle body slip angle may also be determined byusing other methods.

[0052] The control process which is executed in this control unit is nowdescribed in the following with reference to the flow chart of FIG. 4.

[0053] In step ST1, the actual tire steering angle computing unitSTA_(n) receives a steering angle STC from the steering sensor, anddetermines the actual tire steering angles STA_(R) and STA_(L) by takinginto account various design parameters such as the gear ratio of thesteering gear box. In step ST2, a yaw rate YAWR is read out from the yawrate sensor YAWR. In step ST3, a vehicle body lateral acceleration LGand vehicle body fore-and-aft acceleration FG are read out from theacceleration sensors LG and FG before the control flow advances to stepST4.

[0054] In step ST4, the estimated vehicle body speed computing unit VVβcomputes an estimated vehicle body speed (X-component) VVXβ. Thecomputation of the estimated vehicle body speed (X-component) VVXβ maybe carried out as shown in the flow chart shown in FIG. 5.

[0055] Referring to FIG. 5, in step ST4 a, it is determined if theabsolute value of the vehicle body slip angle (side slip angle) β isgreater than a threshold value β_(c). If it is the case (|β|≧β_(c)), theprogram flow advances to step ST4 b where a vehicle body speed changerate VVBG in the traveling direction is computed from the followingequation.

VVBG=(FGEF cos β+LGEF sin β)×KX  (1)

[0056] where KX is a predetermined coefficient which depends on designparameters of the vehicle.

[0057] In step ST4 c, a vehicle body speed change rate VVBG in thefore-and-aft direction is computed from the following equation.

VVXBG=VVBG cos β  (2)

[0058] If the absolute value of the vehicle body slip angle (side slipangle) β is smaller than the threshold value β_(c) in step ST4 a, theprogram flow advances to step ST4 d where the predetermined coefficientKX is multiplied to the estimated fore-and-aft acceleration, or

FGEF×KX  (3)

[0059] In step ST4 e which follows step ST4 c or step ST4 d as the casemay be, the estimated vehicle body speed (X-component) VVXβ is obtainedfrom the following equation

VVXβ(n)=VVXβ(n−1)+VVXBG(n−1)  (4)

[0060] when step ST4 e is reached via step ST4 c, and from the followingequation

VVXβ(n)=VVXβ(n−1)+FGEF×KX  (5)

[0061] when step ST4 e is reached via step ST4 d. Here, (n) indicatesthe current computation loop, and (n−1) indicates the previouscomputation loop.

[0062] The estimated vehicle body speed (X-component) VVXβ is thuscomputed by the subroutine steps ST4 a to ST4 e of step ST4 of the mainroutine, and the estimated vehicle body speed is determined in theestimated vehicle body speed (X-component) computing unit VVXβ.

[0063] Referring to FIG. 2, the estimated vehicle body speed(X-component) computing unit VVXβ and the vehicle body slip angle (sideslip angle) β are forwarded to the estimated vehicle body speedcomputing unit VVβ. The estimated vehicle body speed (X-component) VVXβis computed as described in connection with the subroutine flowdescribed above (steps ST4 a to ST4 e). Therefore, when estimating thevehicle body speed VVβ, the values of the road surface frictionalcoefficient μ, vehicle body slip angle (side slip angle) β, tire slipratios SLP_(mn), brake/traction forces FX_(mn), estimated fore-and-aftacceleration FGE (FGEF), tire lateral forces CF_(mn) and estimatedlateral acceleration LGE (LGEF) are used, and the dynamic state of thevehicle, including the road condition, is represented by a vehiclemodel. Therefore, the estimated vehicle body speed can be determinedaccurately by eliminating the effects of the slippage of the tiresduring braking, road noises and an offset due to the inclination of theroad that may be present when a fore-and-aft acceleration is used.

[0064] As a result, the vehicle body speed can be obtained highlyaccurately even when the vehicle is cornering and a substantial slippageof the tires is present, and the precision of the vehicle motion controlbased on the vehicle speed can be ensured. Once the estimated vehiclebody speed (X-component) VVXβ is obtained, the program flow advances tostep ST5.

[0065] In step ST5, the rolling direction speeds VC_(FR), VC_(FL),VC_(RR) and VC_(RL) of the various wheels are obtained as a basis forthe speeds of the various tires. First of all, the contact point speed(X-component) computing unit VCX_(mn) provides relevant information onthe various tires according to the estimated vehicle body speed(X-component) VVXβ in combination with the estimated yaw rate CFYAWR.This allows the contact point speeds (X-component) VCX_(FR), VCX_(FL),VCX_(RR) and VCX_(RL) to be determined individually for the differenttires. Likewise, the contact point speed (Y-component) computing unitVCY_(mn) allows the contact point speeds (Y-component) VCY_(FR),VCY_(FL), VCY_(RR) and VCY_(RL) to be determined individually for thedifferent tires according to the estimated vehicle body speed(X-component) VVXβ and estimated yaw rate CFYAWR in combination with thevehicle body slip angle β. Therefore, when the vehicle body slip angle βis zero, the contact point speeds (Y-component) are zero.

[0066] Because the contact point speed (X-component) and contact pointspeed (Y-component) are X and Y components of the contact point speed inthe rolling direction of reach tire, the contact point speed(X-component) computing unit VCX_(mn) and contact point speed(Y-component) computing unit VCY_(mn) jointly form a contact pointdirection speed (X- and Y-components) computing unit. The contact pointrolling direction speed computing unit VC_(mn) computes the rollingdirection speeds VC_(FR), VC_(FL), VC_(RR) and VC_(RL) of the individualwheels according to the contact point speeds (X-component) VCX_(mn) andcontact point speeds (Y-component) VCY_(mn).

[0067] In step ST6, the tire slip ratios SLP_(mn) of the individualwheels are obtained. This computation may be based on either the rollingdirection speeds VC_(mn) or wheel speeds VW_(mn). When the computationis based on the rolling direction speeds VC_(mn), SLP_(mn) is determinedas given by the following equation.

SLP _(mn)=100×(VC _(mn) −VW _(mn))/VC _(mn)  (6)

[0068] When the computation is based on the wheel speeds VW_(mn),SLP_(mn) is determined as given by the following equation.

SLP _(mn)=100×(VC _(mn) −VW _(mn))/VW _(mn)  (7)

[0069] In step ST7, the tire (lateral) slip angles α_(mn) are obtainedfrom the contact point speeds (X-component) VCX_(mn) and contact pointspeeds (Y-component) VCY_(mn) before the program flow advances to stepST8 shown in FIG. 6.

[0070] In step ST8, the brake/traction forces (tire fore-and-aft forces)FX_(mn) are computed according to the wheel loads FZ_(mn), tire slipratios SLP_(mn) and road surface frictional coefficient μ that areforwarded to the tire fore-and-aft forces computing unit FX_(mn). Tosimplify the computation, as shown in FIG. 7, the road surfacefrictional coefficient μ may be classified into three levels (high,medium and low) and a corresponding lookup table based on such aclassification of the road surface frictional coefficient μ may beprepared so as to allow brake/traction force (tire fore-and-aft force)coefficients to be determined according to the corresponding tire slipratios. By multiplying the brake/traction force (tire fore-and-aftforce) coefficients, obtained from the table represented in FIG. 7, tothe corresponding wheel loads, the brake/traction forces (tirefore-and-aft forces) for the individual wheels can be determined.

[0071] By using a three dimensional map taking into account theestimated road surface frictional coefficient as shown in FIG. 7, theestimation precision can be improved. The map preferably takes intoaccount three or more levels of the road surface frictional coefficient.

[0072] In step ST9, the estimated fore-and-aft acceleration FGE isobtained from the brake/traction forces (tire fore-and-aft forces)FX_(mn) computed in step ST8. The equation for this computation may beas given in the following.

FGE=(FX _(FR) +FX _(FL) +FX _(RR) +FX _(RL))/(total vehicle weight)  (8)

[0073] In step ST10, the estimated fore-and-aft acceleration FGE isfiltered by an estimated fore-and-aft acceleration filter FGEF. In thiscase, the estimated fore-and-aft acceleration FGE may be filtered simplyby using a low pass filter.

[0074] In step ST11, the tire lateral forces CF_(mn) of the individualwheels are computed according to the wheel loads FZ_(mn), corneringforces FY_(mn) and road surface frictional coefficient μ, in addition tothe tire slip ratios SLP_(mn) that were obtained in step ST6. In thiscase also, to simplify the computation, as shown in FIG. 8, the roadsurface frictional coefficient μ may be classified into three levels(high, medium and low) and a corresponding lookup table based on such aclassification of the road surface frictional coefficient μ may beprepared so as to allow tire lateral force coefficients to be determinedaccording to the corresponding tire slip angles. By multiplying the tirelateral force coefficients to the corresponding wheel loads, the tirelateral forces CF_(mn) for the individual wheels when the tire slipratio is zero can be determined.

[0075] Tire lateral force reduction coefficients are then obtained froma lookup table as shown in FIG. 9 in which the tire lateral forcereduction coefficient is given in relation to the tire slip ratio byclassifying the estimated road surface frictional coefficient μ intothree levels as was the case with the graphs of FIGS. 7 and 8. The tirelateral force reduction coefficient thus obtained according to theparticular tire slip ratio from this three-dimensional graph for eachwheel is then multiplied to the tire lateral force CF_(mn) (for the casewhere the slip ratio is zero) to provide highly accurate values of thetire lateral forces CF_(mn).

[0076] In step ST12, the estimated lateral acceleration LGE is obtainedin the estimated lateral acceleration computing unit LGE from thefollowing equation according to the tire lateral forces obtained fromthe tire lateral force computing unit CF_(mn).

LGE=(CF _(FR) +CF _(FL) +CF _(RR) +CF _(RL))/(total vehicle weight)  (9)

[0077] The estimated yaw rate CFYAWR may be obtained between steps ST12and ST13, and the subroutine for the computation of the estimated yawrate CFYAWR is given in FIG. 10.

[0078] Referring to FIG. 10, in step ST21, the moment MOMFX produced bythe fore-and-aft forces of the tires is computed, for instance,according to the following equation.

MOMFX=(FX _(FR) −FX _(FL))×TRDF+(FX _(RR) −FX _(RL))×TRDR  (10)

[0079] where TRDF and TRDR are the treads of the front and rear wheels,respectively (see FIG. 3).

[0080] In step ST22, the front wheel moment correction coefficient CFK1based on MOMFX of Equation 10 is computed in a front and rear wheelmoment (yaw rate correction coefficient) computing unit CFKx.

CFK 1={LSR×(CF _(FR) +CF _(FL) +CF _(RR) +CF_(RL))+(ΔYRR/KDYR)+MOMFX}/{(LSF+LSR)/(CF _(FR) +CF _(FL))}  (11)

[0081] where LSF is the distance between the gravitational center of thevehicle body and the front axle, LSR is the distance between thegravitational center of the vehicle body and the rear axle (see FIG. 3),and KDYR is a coefficient for converting the actual yaw rate incrementΔYRR into a moment.

[0082] In step ST23, the rear wheel moment correction coefficient CFK2is computed in a similar fashion.

CFK 2={LSF×(CF _(FR) +CF _(FL) +CF _(RR) +CF_(RL))+(ΔYRR/KDYR)+MOMFX}/{(LSF+LSR)/(CF _(RR) +CF _(RL))}  (12)

[0083] In step ST24, the estimated yaw rate increment (estimated yawmoment) ΔYRE based on CFK1 and CFK2 obtained in the preceding steps iscomputed in an estimated yaw rate increment computing unit ΔYREaccording to the following equation.

ΔYRE={LSF×CFK 1×(CF _(FR) +CF _(FL))−LSR×CFK 2×(CF _(RR) +CF_(RL))−MOMFX}×KDYR  (13)

[0084] By integrating the estimated yaw rate increment (estimated yawmoment) ΔYRE, the estimated yaw rate (estimated vehicle body yaw rate)can be obtained.

[0085] In step ST25, the estimated yaw rate CFYAWR(n) of the currentloop of the subroutine is computed in the estimated yaw rate computingunit CFYAWR according to the following equation.

CFYAWR(n)=CFYAWR(n−1)+ΔYRE(n)×Tr  (14)

[0086] where CFYAWR(n−1) is the estimated yaw rate that was obtained inthe previous loop of the subroutine, and Tr is the loop time for thiscomputation. By thus obtaining the estimated yaw rate CFYAWR, theaccuracy of the yaw rate for the motion control, particularly when thevehicle is cornering, can be improved.

[0087] According to the conventional arrangement in which the yaw rateis obtained from the difference between the rotational speeds of theright and left wheels, because the yaw rate value disappears when abrake is applied, the yaw rate value is not available for the yaw momentcontrol at such a time. According to the other conventional arrangementin which the yaw rate is obtained from the lateral forces andfore-and-aft forces of the tires, the accuracy of the estimated tirelateral forces tends to be impaired when the dynamic tire model forestimating the lateral forces and fore-and-aft forces of the tires doesnot correctly model the actual tires, particularly when there aredisturbances from the road surface, and/or when there are errors in thevehicle body slip angle and/or estimated road surface frictionalcoefficient which are highly essential for the vehicle motion control.All these factors contribute to the reduction in the accuracy of theestimated yaw rate.

[0088] On the other hand, according to the foregoing embodiment, notonly the yaw rate YAWR obtained from the yaw rate sensor YAWR is usedbut also the tire slip ratios SLP_(mn), tire slip angles α_(mn) and roadsurface frictional coefficient μ are obtained, and the tire fore-and-aftforces FX_(mn) are computed by using a corresponding dynamic tire model.Further, the front and wheel moment correction coefficients CFK1 andCFK2 are computed from the tire fore-and-aft forces FX_(mn), tirelateral forces FY_(mn) and yaw rate YAWR, and the estimated yaw rateCFYAWR is obtained from the estimated yaw moment ΔYRE computed from thetire fore-and-aft forces FX_(mn), tire lateral forces FY_(mn) and frontand wheel moment correction coefficients CFK1 and CFK2. This allows theestimated yaw rate CFYAWR to accurately reflect the actual vehiclemotion under all conditions, and the problems associated with the priorart such as the loss of accuracy in the estimated yaw rate can beeliminated.

[0089] Even when there are some errors in the estimation of the tirelateral forces FY_(mn), for instance, due to the use of non-standardtires and disturbances from the road surfaces, the errors in the tirelateral forces FY_(mn) and lateral acceleration LGE can be eliminated orminimized by using the front and wheel moment correction coefficientsCFK1 and CFK2, and the adaptability of the dynamic tire model can beimproved.

[0090] According to the prior art, the vehicle body slip angle (sideslip angle) β can be obtained by computing the vehicle body slip angleincrement from the lateral acceleration, vehicle body speed and yawrate, and integrating the thus obtained vehicle body slip angleincrement. However, according to such a conventional approach, whenthere is an offset in the zero point of the yaw rate sensor, because thevehicle body slip angle is obtained by integrating “vehicle body lateralacceleration/vehicle body speed-yaw rate”, the offset is alsointegrated, and an excessive error is produced in the vehicle body slipangle.

[0091] On the other hand, according to the foregoing embodiment, insteadof using the detected value of the yaw rate sensor, the vehicle bodyslip angle increment Δβ is obtained from the lateral acceleration LGE(LGEF), vehicle body speed VVβ and estimated yaw rate CFYAWR, and thevehicle body slip angle β is obtained by adding the vehicle body slipangle increment Δβ to the previously obtained vehicle body slip angleβ(n−1). Thus, even when there is a significant offset in the zero pointof the yaw rate sensor, the accuracy of the vehicle body slip angle βcan be ensured.

[0092] In the control process described above, the maps which take intoaccount the road surface frictional coefficient were used for obtainingthe tire fore-and-aft force coefficient, tire lateral force coefficientand tire lateral force reduction coefficient. In the foregoingembodiment, three possible levels were selected for the road surfacefrictional coefficient (high μ, medium μ and low μ). By thus preparing aplurality of tire models for the different levels of the road surfacefrictional coefficient, the cornering forces, vehicle body fore-and-aftforces (tire lateral and fore-and-aft forces) can be computed by fullytaking into account the actual road surface frictional coefficient andother road surface conditions, and the vehicle body slip angle β can bedetermined at a high precision. The estimated yaw rate CFYAWR can bethus used advantageously as the yaw rate for determining the state ofthe vehicle and executing the motion control of the vehicle.

[0093] In step ST13, the estimated road surface frictional coefficient μis obtained. The estimated road surface frictional coefficient μ can beobtained from the filtered estimated lateral acceleration value LGEF,filtered estimated fore-and-aft acceleration value FGEF and total gripforce TG according to the process flow depicted in the subroutine flowchart of FIG. 11. Referring to FIG. 11, in step ST13 a, it is determinedif the current estimated road surface frictional coefficient μ issmaller than the tire grip force converted value. The tire grip forceconverted value can be represented as a value (TGM/TIRGRP) associatedwith the total grip force TGM. Here, TIRGRP is a conversion coefficientfor changing the dimension of the total grip force TGM to that of theroad surface frictional coefficient. The initial value of the roadsurface frictional coefficient may be 1 which corresponds to a dry roadsurface.

[0094] If the current estimated road surface frictional coefficient μ isequal to or greater than the tire grip force converted value in stepST13 a, the program flow advances to step 13 b to start the process ofestimating the road surface frictional coefficient. In step ST13 b, itis determined if the averaged value of the tire slip angles α_(RR) andα_(RL) of the rear wheels is greater than a threshold value MUSLP. Inthe drawing, these values are given as absolute values because the tireslip angle is either a negative or positive value depending on thedirection of the angle with respect to the neutral direction. When theaverage value of the tire slip angles of the rear wheels is greater thanthe threshold value, the road surface frictional coefficient is obtainedwith respect to a lateral movement. Conversely, when the average valueof the tire slip angles of the rear wheels is equal to or smaller thanthe threshold value, the program flow advances to step ST13 c, and theroad surface frictional coefficient is obtained with respect to afore-and-aft movement.

[0095] In steps ST13 c to 13 e, it is determined if all the conditionsincluding the vehicle speed, tire slip ratios and steering angle thatare required for the computation of the road surface frictionalcoefficient with respect to a fore-and-aft movement are all met.

[0096] In step ST13 c, it is determined if the estimated vehicle bodyspeed (X-component) VVXβ is equal to or greater than a threshold valueVVFGBT. If it is the case, the program flow advances to step ST13 d.Otherwise, the current loop of the subroutine flow is terminated. Instep ST13 d, it is determined if the slip ratio SLP_(mn) of at least oneof the wheels is equal to or greater than a threshold value SLPBT. If itis the case, the program flow advances to step ST13 e. Otherwise, thecurrent loop of the subroutine flow is terminated. In step ST13 d, it isdetermined if the absolute value of the steering angle STC is equal toor greater than a threshold value BTSTC. If it is the case, the programflow advances to step ST13 f. Otherwise, the current loop of thesubroutine flow is terminated.

[0097] In step ST13 f, it is determined if the estimated fore-and-aftacceleration FGE is equal to or greater than the vehicle bodyfore-and-aft acceleration FG obtained from the acceleration sensor. Ifit is the case, the program flow advances to step ST13 g shown in FIG.12 to decrease the road surface frictional coefficient by one notch.Otherwise, the program flow advances to the steps for increasing theroad surface frictional coefficient by one notch. The road surfacefrictional coefficient can be thus obtained, but may also be obtained indifferent ways without departing from the spirit of the presentinvention.

[0098] Referring to FIG. 12, in step ST13 g, it is determined if thevalue obtained by subtracting the vehicle body fore-and-aft accelerationFG obtained from the sensor from the estimated fore-and-aft accelerationFGE is greater than a threshold value BTFG. If it is the case, theprogram flow advances to step ST13 h where it is determined if the stateof exceeding the threshold value BTFG has persisted for more than aprescribed time period. If it is the case, it is determined as a case ofμ jump (a case involving a large change in the road surface frictionalcoefficient), and the program flow advances to step ST13 i. If there isno μ jump, the program flow advances to step ST13 j.

[0099] In step ST 13 j, a certain value such as 0.0078 is subtractedfrom the current road surface frictional coefficient μ to lower theestimated road surface frictional coefficient μ before the program flowadvances to step ST13 k. In step ST13 k, a limiter process is executedwith respect to the upper and lower limits so that the estimated roadsurface frictional coefficient μ may be kept within a prescribed rangeand prevented from reaching an unrealistic value before the current loopof the subroutine flow is terminated.

[0100] In step ST13 i, it is determined if a case of μ jump has beendetected in step ST13 h for more than a prescribed number of times. Theprogram then advances to step ST13 l if a μ jump has occurred for morethan the prescribed number of times, and to step ST13 j if not. In stepST13 l, the tire grip force conversion value (TGM/TIRGRP) is substitutedinto the estimated road surface frictional coefficient μ before theprogram flow advances to step ST13 k. If the value obtained bysubtracting the vehicle body fore-and-aft acceleration FG obtained fromthe sensor from the estimated fore-and-aft acceleration FGE is equal toor smaller than the threshold value BTFG in step ST13 g, the programflow also advances to step ST13 k.

[0101] If the current estimated road surface frictional coefficient μ isfound to be smaller than the tire grip force conversion value in stepST13 a, the program flow advances to step ST13 m. In step ST13 m, theestimated road surface frictional coefficient μ is set as the tire gripforce conversion value (TGM/TIRGRP) before the program flow advances tostep ST13 k.

[0102] If the estimated fore-and-aft acceleration value FGE isdetermined to be less than the vehicle body fore-and-aft acceleration FGdetected by the sensor in step ST13 f, the program flow advances to stepST13 n shown in FIG. 12. In step ST13 n, it is determined if the valueobtained by subtracting the vehicle body fore-and-aft acceleration FGobtained from the sensor from the estimated fore-and-aft accelerationFGE is equal to or smaller than the threshold value BTFG. If this valueis equal to or smaller than the threshold value BTFG, the program flowadvances to step ST13 k and otherwise to step ST13 o. In step ST 13 o, acertain value such as 0.0078 is added to the current estimated roadsurface frictional coefficient μ to raise the estimated road surfacefrictional coefficient μ by one notch before the program flow advancesto step ST13 k.

[0103] In step ST13 b, if the average value of the tire slip angles ofthe rear wheels is determined to be large, and the road surfacefrictional coefficient is therefore to be estimated with respect to alateral movement, the program flow advances to step ST13 p shown in FIG.13. In step ST13 p, it is determined if the actual yaw moment hasexceeded a threshold value. Although this determination process is notshown in FIG. 2, it can be accomplished by forwarding the yaw rate YAWRdetected by the yaw rate sensor 2 to the road surface frictionalcoefficient computing unit μ, and executing a corresponding process inthis unit. If the actual yaw moment is equal to or less than thethreshold value, the current loop of the subroutine flow is terminated.

[0104] If it is determine that the actual yaw moment has exceeded thethreshold value in step ST13 p, the program flow advances to step ST13q. In step ST13 q, it is determined if the estimated lateralacceleration LGE (LGEF) is equal to or grater than the vehicle bodylateral deceleration LG detected by the sensor. If it is the case, theprogram flow advances to step ST13 r to lower the estimated road surfacefrictional coefficient μ. Otherwise, the program flow advances to thesteps for raising the estimated road surface frictional coefficient μ.

[0105] In step ST13 r, it is determined if a value obtained bysubtracting the vehicle body fore-and-aft acceleration LG obtained fromthe sensor from the estimated lateral acceleration LGE is greater thanthe threshold value BTLG. If it is the case, the program flow advancesto step ST13 s.

[0106] In step ST13 s, it is determined if the state of exceeding thethreshold value as determined in step ST13 r has persisted for more thana prescribed period of time. If it is the case, the program flowadvances to step ST13 t as a case of a μ jump. Otherwise, the programflow advances to step ST13 u.

[0107] In step ST13 u, a certain value such as 0.0078 is subtracted fromthe current road surface frictional coefficient μ to lower the estimatedroad surface frictional coefficient μ before the program flow advancesto step ST13 v. In step ST13 v, a limiter process is executed withrespect to the upper and lower limits so that the estimated road surfacefrictional coefficient μ may be kept within a prescribed range andprevented from reaching an unrealistic value before the current loop ofthe subroutine flow is terminated.

[0108] In step ST13 t, it is determined if a case of μ jump has beendetected in step ST13 s for more than a prescribed number of times. Theprogram then advances to step ST13 w if a μ jump has occurred for morethan the prescribed number of times, and to step ST13 u if not. In stepST13 w, the tire grip force conversion value (TGM/TIRGRP) is substitutedinto the estimated road surface frictional coefficient μ before theprogram flow advances to step ST13 v. If the value obtained bysubtracting the vehicle body lateral acceleration LG obtained from thesensor from the estimated lateral acceleration sensor LGE is equal to orsmaller than the threshold value BTLG in step ST13 r, the program flowalso advances to step ST13 v.

[0109] If the estimated lateral acceleration value LGE (LGEF) isdetermined to be less than the vehicle body lateral acceleration LGdetected by the sensor in step ST13 q, the program flow advances to stepST13 x. In step ST13 x, it is determined if the value obtained bysubtracting the vehicle body lateral acceleration LG obtained from thesensor from the estimated lateral acceleration LGE (LGEF) has exceededthe threshold value BTLG. If the threshold value has been exceeded, theprogram flow advances to step ST13 y where a certain value such as0.0078 is added to the current estimated road surface frictionalcoefficient μ to raise the estimated road surface frictional coefficientμ by one notch before the program flow advances to step ST13 v.

[0110] By thus obtaining the estimated road surface frictionalcoefficient μ, the accuracy of the road surface frictional coefficient μcan be improved. According to the prior art, the road surface frictionalcoefficient μ tended to be estimated higher than it actually is when theestimation relied only on the tire slip ratios SLP_(mn) and the tireslip ratios are relatively small as is the case on snow covered roads orfrozen roads. However, the logic of obtaining the estimated road surfacefrictional coefficient μ which is described above is free from suchproblems.

[0111] More specifically, according to the embodiment described above,the estimation can be carried out by using the vehicle body fore-and-aftand lateral accelerations in such a manner as to fully take into accountthe fact that the vehicle is traveling straight or cornering. Also, bydetecting a μ jump, the estimated road surface frictional coefficient μis prevented from deviating significantly from the actual value, and avalue which agrees with or close to the actual road surface frictionalcoefficient μ can be obtained at all times. Also, the fore-and-aftaccelerations, lateral accelerations and yaw moment can be estimatedfrom the tire data maps (FIGS. 7 and 8) for determining the tire slipangles α_(mn), tire lateral forces and tire fore-and-aft forces, and theestimated road surface frictional coefficient μ can be corrected oradjusted by comparing these estimated values with the values actuallydetected by the sensors (to effect the raising and lowering of μ whichwas described above). Based on the adjusted estimated road surfacefrictional coefficient μ, the gains of the tire data maps can beadapted. In the illustrated embodiment, the tire data maps are adaptedto three levels of the road surface frictional coefficient μ.

[0112] Thus, the road surface frictional coefficient μ can be estimatedat a high precision without regard to the condition of the vehicle, inparticular without regard to if the vehicle is accelerating,decelerating, traveling straight or cornering. The precision of the roadsurface frictional coefficient μ can be as high as 0.05 if the precisionof the tire slip angles SLP_(mn) is in the order of 0.5 degrees.

[0113] In step ST14, the vehicle body slip angle increment (side slipangle increment) Δβ is computed in the vehicle body slip angle incrementcomputing unit Δβ according to the following equation,

Δβ=KLGVXD×(LGE/VVβ)−CFYAWR  (15)

[0114] where KLGVXD is a coefficient for conforming the dimension of thequotient of the estimated lateral acceleration LGE (LGEF) to theestimated vehicle body speed VVβ to that of the estimated yaw rateCFYAWR. The vehicle body slip angle (side slip angle) β can be obtainedby integrating the vehicle body slip angle increment (slide slip angleincrement) Δβ.

[0115] In step ST15, the vehicle body slip angle (side slip angle) β iscomputed in the vehicle body slip angle (side slip angle) computing unitβ according to the following equation.

β(n)=β(n−1)+Δβ(n)  (16)

[0116] where (n) indicates the value computed in the current loop and(n−1) indicates the value computed in the previous loop. In other words,the vehicle body slip angle β is computed by adding the vehicle bodyslip angle increment Δβ obtained in step ST14 of the current loop to thevehicle body slip angle β(n−1) obtained in the previous loop.

[0117] The computation of the vehicle body slip angle β described aboveis based on a tire model which yields the tire fore-and-aft forces andtire lateral forces by fully taking into account the different levels(high, medium and low) of the road surface frictional coefficient, andcan produce the vehicle body slip angle β at a high precision byminimizing the errors in the estimated values of the tire lateral forcesand tire fore-and-aft forces (brake/traction forces).

[0118] The control system having the structure described above canperform a control action which provides both a high stability and afavorable steerability. As an example of such a control action, thecontrol process in cases of oversteer and understeer is described in thefollowing.

[0119] According to a conventional arrangement, a control variable isobtained from the values of the steering angle and yaw rate that aredetected by using sensors, and the moment of the vehicle body iscontrolled by selectively activating the brakes of the different wheelsin appropriate manners in cases of oversteer and understeer. Morespecifically, the control variable is given as a deviation of the yawrate as detected by a yaw rate sensor from a standard yaw ratedetermined from the steering angle, and the vehicle body moment iscontrolled in such a manner that the outer wheels are braked in case ofundersteer and the inner wheels are braked in case of oversteer.However, according to such a yaw rate control, because the road gripconditions of the four wheels are not directly monitored, the yaw momentmay be controlled, but it does not mean that the travel path of thevehicle is controlled. For instance, the travel path of the vehicle mayswerve outward while the vehicle turns a corner (drift out).

[0120] On the other hand, according to the control system of the presentinvention described above, the estimated vehicle body speed VVβ, roadsurface frictional coefficient μ, vehicle body slip angle β and tireslip angles α_(mn) are estimated, and the two rear wheels and outerfront wheel are braked in a controlled manner in dependence on thevehicle body slip angle β in the case of oversteer while the two rearwheels are braked in a controlled manner in case of understeer independence on the degree of understeer. By thus controlling the vehiclebody moment and reducing the kinetic energy of the vehicle, undesiredmotions of the vehicle such as a drift out can be avoided.

[0121] An exemplary control process at the time of oversteer/understeeris described in the following with reference to the flow charts of FIGS.14 to 16 and the control logic block diagram of FIG. 17. Referring toFIG. 14, in step ST31, it is determined if the momentum reductioncontrol is in progress. As the control process at the time ofoversteer/understeer requires at least the two rear wheels to be braked,it is desirable to determine if the rear wheels are being braked. If itis not the case, the program flow advances to step ST32 to start thiscontrol process. If the rear wheels are being braked, the program flowadvances to step ST33 to execute the process of terminating the momentumreduction control.

[0122] In step ST32, as a condition for starting the momentum reductioncontrol, it is determined if the estimated vehicle body speed VVβ isequal to or greater than a threshold value VVALST (20 km/h, forinstance). If it is the case, the program flow advances to step ST34 todetermine if there is a slippage exceeding a prescribed level existingfor each of the wheels. In step ST34, it is determined if the absolutevalue of the tire slip angle α_(Rn) of either one of the rear wheels isequal to or greater than a threshold value ALFIN. If it is the case, theprogram flow advances to step ST35.

[0123] The condition for starting the momentum reduction control isdetermined from the vehicle speed and the tire slip angles of the rearwheels. If the variable is below the corresponding threshold value instep ST32 or ST33, the current loop of the subroutine is terminated.

[0124] In step ST35, the basic target rear wheel speeds VI_(Rn) are thatare required for the momentum reduction control of the vehicle arecomputed. Referring to FIG. 17, the basic target rear wheel speedsVI_(Rn) are computed by computing a target wheel speed modificationratio RUDVR from the standard yaw rate MYRNO obtained from the steeringangle STC and yaw rate YAWR, computing the rolling direction speedsVC_(mn) from the estimated vehicle body speed (X-component) VVXβ, andthen computing the basic target rear wheel speeds VI_(Rn) from thetarget wheel speed modification ratio RUDVR and rolling direction speedsVC_(mn).

[0125] In step ST33, it is determined if the vehicle speed condition ismet by comparing the vehicle speed with a threshold vehicle speed (10km/h, for instance) that can be safely considered as a substantiallystationary state. If the vehicle is in a substantially stationary state,the current loop of the subroutine is terminated as a case of thevehicle speed condition having been met for terminating the momentumreduction control. Otherwise, the program flow advances to step ST36 ascase of the vehicle speed condition not having been met. In other words,the vehicle momentum control is continued only if the momentum of thevehicle is not insignificant.

[0126] In step ST36, the vehicle body slip angle condition is evaluatedby comparing the vehicle body slip angle with a threshold value. If thevehicle body slip angle is equal to or greater than a threshold value,the program flow advances to step ST37. In other words, if the vehiclebody slip angle becomes excessive during the rear wheel control, themomentum control is terminated so as to prevent the vehicle motion tobecome unstable (oversteer or spin).

[0127] In step ST37, it is determined if the tire slip angle conditionis met by comparing the tire slip angle with a threshold value. If thetire slip angle is equal to or greater than the threshold value (whichindicates the regaining of the stable condition), the program flowadvances to step ST38. In step ST38, it is determined if the lateralacceleration condition is met by comparing the lateral acceleration witha threshold value. If the lateral acceleration is equal to or greaterthan the threshold value (which indicates the regaining of the stablecondition), the program flow advances to step ST39. Otherwise, theprogram flow advances to step ST35 to continue the ongoing momentumreduction control. If the vehicle body slip angle condition is met instep ST36, or if the tire slip angle condition is met in step ST37, theprogram flow advances to step ST39.

[0128] In step ST39, it is determined if a delay time (200 ms, forinstance), by elapsing of which indicates that the momentum reductioncontrol may be terminated, has expired. If the delay time has notexpired, the program flow advances to step ST35 to continue the ongoingmomentum reduction control. Otherwise, the current loop of thesubroutine is terminated. Thus, the vehicle momentum control iscontinued as long as a certain condition is met, and would not beterminated in a short period of time once it is started.

[0129] If the program flow has advanced to step ST35, it is determinedin the following step ST40 if it is a case of oversteer or understeer.This determination step may be based on the deviation between thestandard yaw rate MYRNO and yaw rate YAWR, and is executed in the targetwheel speed modification ratio computing unit RUDVR (O/U). Depending onthe output of the target wheel speed modification ratio computing unitRUDVR which serves as means for detecting oversteer and understeer, theprogram flow advances to step ST41 in case of understeer or ST42 in caseof oversteer.

[0130] In step ST41, the target vehicle speed modification decrement(the target vehicle speed modification ratio RUDVR) which is computed independence on the degree of understeer is subtracted from the referencetarget wheel speeds (rolling direction speeds VC_(mn)) to yield thecontrol target wheel speeds (vehicle momentum reduction control targetvehicle speeds) VI_(Rn). In step ST43 (see FIG. 15), the target controlvariable (VE_(RR)/VE_(RL)) for the inner wheel is computed from thedeviation of the corresponding wheel speed (VW_(RR)/VW_(RL)) from thecontrol target wheel speed VI_(Rn). Likewise, in step ST44, the targetcontrol variable for the outer wheel is computed from the deviation(VE_(RR)/VE_(RL)) of the corresponding wheel speed (VW_(RR)/VW_(RL))from the control target wheel speed VI_(Rn).

[0131] The limiter value ILIN for the inner side is obtained from thetire lateral forces CF_(mn), tire fore-and-aft forces FX_(mn) and roadsurface frictional coefficient μ in step ST45 (see FIG. 17), and thelimiter value ILOUT for the outer side is similarly obtained in stepST46.

[0132] When the program flow had advanced to step ST42 as a case ofoversteer, the wheel control variables (target control variables) arecomputed in this step. The reference target wheel speeds (rollingdirection speed VC_(mn)) are set as the control target wheel speeds(vehicle momentum reduction control target vehicle speeds) VI_(Rn), andthe target control variable (VE_(RR)/VE_(RL)) for each of the right andleft rear wheels is computed from the deviation of the correspondingcontrol target wheel speed VI_(Rn) from the detected wheel speed of thecorresponding rear wheel (VW_(RR)/VW_(RL)).

[0133] In step ST47, the limiter value ILTOT is obtained similarly as inthe case of the understeer, and the program flow advances to step ST48.The program flow also advances from step ST46 to step ST48.

[0134] In step ST48, the upper limit of the rear wheel control variableis set by using the limiter value computed in step ST46 or ST47, and therear wheel control variable (vehicle momentum reduction controlvariable) IT_(Rn) which is limited within a prescribed range isobtained.

[0135] Step ST49 starts the process of computing a control variable bywhich the moment control based on the moment obtained from the vehiclebody slip angle β is restricted, in case of oversteer.

[0136] In step ST49, it is determined if the moment (vehicle body slipangle) control is in progress. It can be determined from the magnitudeof the vehicle body slip angle β. If not, the program flow advances tostep ST50 to determine if the condition to start the moment control ismet. In step ST49, if the moment control is in progress, the programflow advances to step ST51 to start the process of determining if thecondition to terminate the moment control is met.

[0137] In step ST50, it is determined if the process of estimating thesteering angle STC has completed. If this estimation process hascompleted, the program flow advances to step ST52. Otherwise, theprocess of determining if the condition to start the moment control ismet is terminated. In step ST52, it is determined if the estimatedvehicle body speed VVβ is equal to or greater than a threshold valueVVALST (20 km/h, for instance). If it is the case, the program flowadvances to step ST53. In step ST53, it is determined if the tire slipangles α_(Rn) meet the condition for terminating the moment control.This condition is met when the absolute value of the tire slip anglesα_(Rn) is equal to or greater than a threshold value ROTIN1 and the tireslip angle change rate Dα (degrees/sec) is equal to or greater than athreshold value DROTIN, or when the absolute value of the tire slipangles α_(Rn) is equal to or greater than a threshold value ROTIN2. Ifthis condition is met, the program flow then advances to step ST54 (seeFIG. 16).

[0138] In step ST54, it is determined if the vehicle body slip angle βand vehicle body slip angle increment Δβ meet the prescribed condition.This condition is met when the vehicle body slip angle β is equal to orgreater than a threshold value β1 and the vehicle body slip angle changerate Dβ (degrees/sec) is equal to or greater than a threshold value Dβor when the vehicle body slip angle β is equal to or greater than athreshold value β2 (>β1). When this condition is met, the programadvances to step ST55.

[0139] In step ST50, and steps ST52 to ST54, it is determined if thecondition for starting the moment control is met. If it is determined inany of these steps that the condition for starting the moment control isnot met, the current loop of the subroutine is terminated.

[0140] In step ST51, it is determined if the vehicle speed meets theprescribed condition for terminating the moment control. If the vehiclespeed is equal to or lower than a lower speed limit (10 km/h, forinstance), the current loop of the subroutine is terminated because thecondition for terminating the moment control has been met. Otherwise,the program flow advances to step ST56 as a case of the condition notbeing met. In step ST56, it is determined if the vehicle slip angle andyaw rate have a same sign. If it is the case or they have a same sign,the program flow advances to step ST57.

[0141] In step ST57, it is determined if the tire slip angle conditionhas been met (indicating that a stable condition has been restored). Ifthis condition is met, the program flow advances to step ST58.Otherwise, the program flow advances to step ST55. Also if the signs ofthe vehicle body slip angle and yaw rate differ from each other in stepST56, the program flow advances to step ST58.

[0142] In step ST58, it is determined if a delay time (200 ms, forinstance, by elapsing of which the vehicle body slip angle control maybe terminated) has expired. If the delay time has not expired, theprogram flow advances to step ST55 to continue the vehicle body slipangle control. Otherwise, the current loop of the subroutine isterminated.

[0143] In step ST55, because the condition for the vehicle body slipangle has been met, the target vehicle body slip angle ROTA for thiscontrol is computed from the road surface frictional coefficient μ asshown in FIG. 16. In step ST59, the moment control variables (VEβ, dVEβand d²VEβ) are computed from the deviation between the target vehiclebody slip angle ROTA and vehicle body slip angle β. Here, VEβ is a valueobtained by subtracting the target vehicle body slip angle β (ROTALM)from the estimated vehicle body slip angle β, and indicates thedeviation from the target vehicle body slip angle (limit angle). dVEβ isan increment (or a differential value) of this variable, and correspondsto the yaw rate when the target vehicle body slip angle β is a fixedvalue. d²VEβ is a change rate of the change rate (a second orderdifferential value of the deviation), and corresponds to the yaw rateincrement when the target vehicle body slip angle β is a fixed value. Instep ST60, the limiter values (ROTLH and ROTLL) of these controlvariables are computed according to the road surface frictionalcoefficient μ and wheel loads FZ_(mn). In step ST61, the moment controlvariables (VEβ, dVEβ and d²VEβ) are limited by the limiter values (ROTLHand ROTLL) and the outer front wheel control variable ROTTOTL isdetermined before this routine is terminated.

[0144] Depending on if oversteer or understeer is detected by the targetwheel speed modification ratio computing unit RUDVR as well as the rearwheel control variable IT_(Rn) and outer front wheel control variableROTTOTL, the target hydraulic pressures of the four wheels for the brakecontrol are determined in a target hydraulic pressure determining unitP_(mn), and the corresponding brake forces of the four wheels areproduced by a brake force control unit VEU. The target hydraulicpressure determining unit P_(mn) and brake force control unit VEUjointly form the means for controlling the brake force which not onlyreduces the kinetic energy of the vehicle but also controls the vehiclebody moment, and controls the drift out of the vehicle.

[0145] For instance, as shown in FIG. 18, when the neutral travel pathof a vehicle turning a corner is given by the solid line arrow N in thedrawing, the travel path would be given as indicated by the broken linearrow US in the case of understeer, and as indicated by the imaginaryline arrow OS in the case of oversteer.

[0146] In the case of understeer, brake forces FX_(RR) and FX_(RL) areapplied to the rear wheels RR and RL according to target hydraulicpressures P_(RR) and P_(RL) which are in turn determined according tothe dynamic state of the vehicle and road surface condition as describedabove. This allows the vehicle (which otherwise has an understeertendency) to travel along the neutral cornering path. In the case ofoversteer, not only brake forces FX_(RR) and FX_(RL) are applied to therear wheels RR and RL according to target hydraulic pressures P_(RR) andP_(RL) but also a brake force FC_(FR) is applied to the outer frontwheel (right front wheel FR in the illustrated embodiment) according toa target hydraulic pressures P_(FR). This again allows the vehicle(which otherwise has an oversteer tendency) to travel along the neutralcornering path.

[0147] When an attempt is made to regain the vehicle from a stateinvolving a large vehicle body slip angle to a more stable state eithermanually or automatically, according to the prior art, the inertia ofthe vehicle body tends to cause an overshoot in the opposite direction,and it could cause an unstable oscillating movement of the vehicle. Thecontrol process for preventing such an overshoot and ensuring a stablemovement to the vehicle is described in the following.

[0148] In such a vehicle motion control, the vehicle body slip anglecontrol logic shown in FIG. 19 and the inertia moment control logicshown in FIG. 20 may be used. Referring to FIG. 19, to control thevehicle body slip angle β, first of all, various estimated values suchas the estimated vehicle body speed (X-component) VVXβ, vehicle bodyslip angle β, estimated yaw rate CFYAWR, estimated lateral accelerationLGE and estimated fore-and-aft acceleration FGE are determined from theoutput values of the various sensors in a value estimating block 11.These estimated values and output values of the sensors are forwarded toa vehicle motion momentum control block 12, a vehicle body slip anglecontrol block 13 and inertia moment control block 14. The valueestimating block 11 is incorporated with a counter steer determiningunit CS.

[0149] In the vehicle motion momentum control block 12, the estimatedvehicle body speed (X-component) VVXβ, basic target rear wheel speedsVI_(Rn), vehicle momentum reduction control variables IN_(mn), tire slipangles α_(mn), target wheel speed modification ratio RUDVR and limitervalues IL are obtained. The vehicle momentum reduction control variablesIN_(mn) each consist of a deviation IN of the actual fore-and-aft forceof each of the rear wheels that is controlled from the targetfore-and-aft force of the corresponding rear wheel, and is in fact a sumINETO_(mn) of a proportional term INP_(mn), integral term INI_(mn) anddifferential term IND_(mn). The limiter values IL include the limitervalue INELMOUT for the outer rear wheel in case of understeer, limitervalue INELMIN for the inner rear wheel in case of understeer, limitervalue INELMRR for the right rear wheel in case of non-understeer, andlimiter value INELMRL for the left rear wheel in case of non-understeer.The vehicle memntum reduction control values IN_(mn) are given as finalvalues that are subjected to a limiter process.

[0150] In the vehicle body slip angle control block 13, the estimatedvehicle body speed (X-component) VVXβ, tire slip angles α_(mn), vehiclebody slip angle control variable ROT and limiter value ROTL for thecontrol variable are obtained. The vehicle body slip angle controlvariable ROT consists of ROTPBT, ROTIBT, ROTDBT, ROTDDBT and ROTTOL, andthe control value limiter value ROTL consists of ROTLH and ROTLL.

[0151] In the inertia moment control block 14, the estimated vehiclebody speed (X-component) VVXβ, yaw moment ΔYRR/ΔYRE, inertia momentcontrol variable YRRTO and limiter values ROTL for this control variableare obtained. The control logic in the inertia moment control block 14is described in the following with reference to FIG. 20.

[0152] As shown in FIG. 20, the steering angle STC, vehicle body slipangle β and estimated vehicle body speed (X-component) VVXβ areforwarded to an inertia moment control target deviation computing unitYEYRR. The actual yaw rate increment ΔYRR is also forwarded to theinertia moment control target deviation computing unit YEYRR. Based onthe road surface frictional coefficient μ, an inertial moment controlstart threshold value YRRSTT is obtained in an inertial moment controlstart threshold value computing unit YRRSTT, and an inertial momentcontrol end threshold value YRREN is obtained in an inertial momentcontrol end threshold value computing unit YRREN. An inertia momentcontrol target deviation YEYRR is obtained in an inertia moment controltarget deviation computing unit YEYRR according to the values STC, β,VVXβ, ΔYRR, YRRSTT and YRREN.

[0153] Control variable limiter values ROTLH and ROTLL are obtained in acontrol variable limiter value computing unit ROTL according to the roadsurface frictional coefficient μ and wheel loads Z_(mn). An inertiamoment control variable YRRTO is obtained in an inertia moment controlvariable computing unit YRRTO according to the inertia moment controltarget deviation YEYRR and limiter values ROTL. Target hydraulicpressures P_(Fn) for the front wheels are obtained in a target hydraulicpressure computing unit P_(mn) according to the inertia moment controlvariable YRRTO.

[0154] Referring to FIG. 19, in the vehicle body slip angle controllogic, the values from the vehicle motion momentum control block 12,vehicle body slip angle control block 13 and inertia moment controlblock 14 are forwarded to a wheel control unit 15. The wheel controlunit 15 then computes control wheel selection values CFNC_(mn) andtarget hydraulic pressures P_(mn) according to these values, and executea control process for producing a corresponding brake force in each ofthe wheels that are the subject of the control process.

[0155] The motion control of a vehicle which is steered to the right andleft in succession is now described in the following with reference tothe flow chart of FIG. 21, and diagrams of FIGS. 22 and 23. This controlprocess is typically executed as a subroutine process.

[0156] In step ST71, the vehicle body slip angle β, yaw rate YAWR andsteering angle STC are read out. In step ST72, it is determined if acontrol flag (FLAG) is on. If the control flag FLAG is on (which meansthat the motion control process is in progress), the program flowadvances to step ST73 to determine the upper limit brake force for theouter front wheel. This upper limit brake force is determined as a brakeforce that can produce a maximum brake force in the outer front wheelwithout locking it. The outer wheel is determined as the right frontwheel when the steering angle STC indicates a leftward turn and as theleft front wheel when the steering angle STC indicates a rightward turn.

[0157] In step ST74, it is determined if the yaw rate YAWR isdecelerating. If the yaw rate YAWR is decelerating, the program flowadvances to step ST75 to reset the flags. If the yaw rate YAWR is notdecelerating in step ST74, the current loop of the subroutine isterminated.

[0158] The deceleration of the yaw rate YAWR is determined if theproduct of the yaw rate YAWR and actual yaw rate increment ΔYRR isnegative (YAWR×ΔYRR<0) and the absolute value of the actual yaw rateincrement is equal to or smaller than a threshold value (thresholdvalue≧|ΔYRR|). Therefore, when the sign of the actual yaw rate incrementΔYRR has changed, the control process is terminated.

[0159] If the control flag FLG is off (the motion control has notstarted) in step ST72, the program flow advances to step ST76 where itis determined if a condition flag (condition flag) is on. If thecondition FLAG is on (monitoring), the program flow advances to stepST77, and execute the monitoring and controlling process in steps ST77and ST78.

[0160] In step ST77, it is determined if the sign of the yaw rate YAWRhas changed. This can be accomplished by determined if the product ofthe yaw rate YAWR(n) of the current loop and the yaw rate YAWR(n−1) ofthe preceding loop (YAWR(n)×YAWR(n−1)<0) has become negative.

[0161] Suppose a case where a vehicle travels an S-curve as shown inFIG. 22 and is required to be steered successively. When a counter steeraction is taken in the state indicated by I in the drawing, a yaw rateYAWR (denoted by γ in the drawing) is produced, and a substantial actualyaw rate increment ΔYRR (denoted by Δγ in the drawing) is produced. Ifthe yaw rate YAWR is zero and a maximum vehicle body slip angle βmaxoccurs at a point indicated by II in FIG. 22, this is the point at whichthe sign of the yaw rate YAWR reverses.

[0162] When this vehicle undergoing this motion is viewed from above, itcan be viewed as undergoing a swing motion as shown in FIG. 23. In thedrawing, the vehicle is traveling upward in the drawing, and thedirection of the vehicle speed V agrees with the traveling direction.Therefore, the state involving the maximum vehicle body slip angle βmaxwill be as shown in the right end (II) of the drawing.

[0163] In step ST77, if the sign of the yaw rate YAWR has reversed, theprogram flow advances to step ST78. Otherwise, the current loop of thesubroutine is terminated. In step ST78, it is determined if the absolutevalue of the actual yaw rate increment ΔYRR is equal to or greater thana threshold value γ_(a). If it is the case, the program flow advances tostep ST79 to set up the control flag (control FLAG) and start thecontrol process before the current loop of the subroutine is terminated.

[0164] In the subsequent loop of the control process, the program flowadvances from step ST72 to step ST73. Therefore, because the front wheelis steered to the right in the illustrated embodiment as indicated byIII in FIG. 22, owing to the control action executed in step ST73, abrake force FX_(FL) is produced in the left front wheel. This reducesthe vehicle body slip angle β, and stabilizes the state of the vehicle.

[0165] When the condition FLAG is determined to be off (stablecondition) in step ST76, or when the absolute value of the actual yawrate increment ΔYRR is smaller than a threshold value γ_(a) in stepST78, the program flow advances to step ST80. In steps ST80 and ST81, apresence of an unstable condition is determined.

[0166] In step ST80, it is determined if a counter steer action is beingtaken in the counter steer determining unit CS. This can be accomplishedby determining if the product of the yaw rate YAWR and steering angleSTC is negative (YAWR×STC<0). If a counter steer condition is detected,the program flow advances to step ST81. If a counter steer condition isnot detected, the program flow advances to step ST82 to turn off thecondition FLAG because the vehicle is in a stable state.

[0167] In step ST81, it is determined if the absolute value of thevehicle body slip angle β is equal to or greater than a threshold valueβ_(a). If it is the case, the program flow advances to step ST83 to turnon the condition FLAG because the vehicle is in an unstable state. Morespecifically, if the absolute value of the vehicle body slip angle β isequal to or greater than a threshold value β_(a) and a counter steeraction is detected, the state is considered as hazardous. After goingthrough steps ST82 and ST83, the current loop of the subroutine isterminated.

[0168] If the condition FLAG is turned on in step ST83, the program flowadvances to step ST77 in the next loop of the control process. If anunstable condition is detected in step ST78 which follows step ST77, thecontrol FLAG is turned on in step ST79. Therefore, step ST73 is executedin the subsequent loop of the control process. In this case also, abrake force is produced in the outer front wheel, and the vehicle bodyslip angle β is reduced. This stabilizes the state of the vehicle, andthe vehicle travels as indicated by IV and V in FIG. 22.

[0169] According to the conventional motion control, the control actionstarts as the state indicated by the imaginary lines moves to the stateindicated by IV in FIG. 23, and the brake force is produced only in thestate indicated by IV in FIG. 22. Therefore, there is a certain delay inthe control action.

[0170] On the other hand, according to the motion control of the presentinvention, the condition for starting the control action is based on thedetection of a counter steer action, yaw rate YAWR and actual yaw rateincrement ΔYRR. Therefore, as soon as the maximum vehicle body slipangle βmax has been reached, the increases in the yaw rate and actualyaw rate increment ΔYRR at the next corner can be predicted, monitoredand controlled, and this ensures a stable state of the vehicle.

[0171] In the foregoing embodiment, the control action was based on thedistribution of a brake force to the different wheels, but it is alsopossible to distribute a drive force or traction to the different wheelsto achieve a similar goal. In such a case, drive force distributingsystem will be required for distributing the drive force to thedifferent wheels.

[0172] Thus, according to the foregoing embodiment, when a counter steeraction is taken while the vehicle corners, the outer front wheel isbraked in a controlled manner upon detecting that the vehicle body slipangle and actual yaw rate increment are equal to or greater thanthreshold values. Therefore, even when a vehicle undergoes a swingingmotion with the vehicle body slip angle reaching a maximum and beingreduced thereafter, because the control system predicts and monitors theincrease in the yaw rate and actual yaw rate increment before themaximum vehicle body slip angle has been reached, the vehicle is enabledto travel in a stable manner. In particular, even when the vehicletravels an S-curve and successive steering actions take place, thevehicle can travel in a stable manner.

[0173] Although the present invention has been described in terms of apreferred embodiment thereof, it is obvious to a person skilled in theart that various alterations and modifications are possible withoutdeparting from the scope of the present invention which is set forth inthe appended claims.

1. A vehicle motion control system for controlling a motion of a vehicleduring cornering, comprising: a steering angle sensor for detecting asteering angle; a yaw rate sensor for detecting an actual vehicle yawrate; an actual yaw rate increment computing unit for computing anincrement of said actual yaw rate; a vehicle body slip angle computingunit for estimating a vehicle body slip angle; a brake force computingunit for controlling brake forces of right and left front wheels of saidvehicle; and a counter steer detecting unit for detecting a countersteer action according to the steering angle and yaw rate; said brakeforce computing unit being adapted to brake an outer front wheel in acontrolled manner when a sign of said yaw rate has changed and said yawrate increment has become equal to or greater than a threshold valueafter a counter steer action has been detected and said vehicle bodyslip angle has become equal to or greater than a threshold value.
 2. Avehicle motion control system according to claim 1, wherein said countersteer detecting unit is adapted to detect a counter steer action whensigns of said steering angle and yaw rate differ from each other.